Reduction and control of trivalent chromium in hexavalent chromium processing solutions

ABSTRACT

A PROCESS FOR REDUCING THE QUANTITY OF TRIVALENT CHROMIUM IN HEXAVALENT CHROMIUM PROCESSING SOLUTIONS, WHICH INVOLVES AGITATING A SOLUTION CONTAINING THE TRIVALENT CHROMIUM WITH LEAD DIOXIDE AT A TEMPERATURE OF AT LEAST 40*C. AND SEPARATING THE RESULTING INSOLUBLE LEAD COMPOUNDS FROM THE SOLUTION.

United States Patent US. Cl. 20451 laims ABSTRACT OF THE DISCLOSURE Aprocess for reducing the quantity of trivalent chromium in hexavalentchromium processing solutions, which involves agitating a solutioncontaining the trivalent chromium with lead dioxide at a temperature ofat least 40 C. and separating the resulting insoluble lead compoundsfrom the solution.

BACKGROUND OF THE INVENTION Field of the invention Trivalent chromiumCr(III) is often present as an unwanted contaminant in hexavalentchromium processing solutions, as for example, chromium electroplating(plating) solutions, and chromic acid anodizing solutions. Generally,the undesirable trivalent chromium is formed in situ by chemical and/ orelectrochemical reduction of the hexavalent chromium and is detrimentalto the process and/or product if not at least partially removed oroxidized to the hexavalent state.

Important among those hexavalent chromium proccessing solutionscontaminated with trivalent chromium are the plating solutions or bathsused for the electrodeposition of metallic chromium andchromium-containing electroplates, for various concentrations oftrivalent chromium Cr(III) are found in all operating chromium platingbaths.

Low concentrations of Cr(IH) do not interfere with the satisfactoryoperation of such baths or adversely affect the quality of the chromiumdeposits produced. Excessive concentrations of Cr(III), however, cansignificantly increase the power requirements and also lead to a poorchromium plate. It is apparent, therefore, that the Cr(IH) concentrationin chromium plating baths should be maintained below certain limits.

Chromium plating techniques differ from those of most other platingprocedures in that the anodes employed are not composed of thedepositing metal. In chromium plating the anodes are frequentlyfabricated of lead or of lead alloyed with antimony and/ or tin.

The presence of lead in the anode and the relation between the surfacearea of the anode to that of the cathode, significantly affect theconcentration of Cr(III) in the bath. It has, in fact, long been knownthat the Cr(III) concentration can effectively be controlled in achromium plating bath by maintaining a proper ratio of lead anode 'tocathode surface area.

The mechanism responsible for controlling the concentration of Cr(III)is dependent on the formation of a film of lead dioxide (PbO- which isdeposited on the surface of the anode during the plating period. Leadand lead alloy anodes are insoluble in chromium plating baths and thePbO film which forms, oxidizes Cr(IH) to hexavalent chromium Cr(VI). Thegreater the anode surface area, the greater the amount of PbO whichforms, and consequently the more Cr(III) that is oxidized. Thus, underideal conditions, the anode surface area can be adjusted to maintain theCr(III) concentration within proper limits.

Chromium plating is usually applied to articles that permit the use of arelatively large ratio of anode area to cathode area. In the field ofdecorative plating, for example, conventional anodes are frequently usedand inherently provide a high ratio of anode to cathode area, often ashigh as about four to one. Under these conditions the concentration oftrivalent chromium is kept relatively low when a steady state isestablished between the reduction of Cr(VI) to Cr(III) at the cathode,and the oxidation of Cr(III) to Cr(VI) at the anode. Plating on theinside of tubes or cylinders, however, necessarily involves a smallerratio of anode to cathode area than that normally employed in decorativeplating. When plating with inside anodes where the surrounding workpiece to be plated is the cathode, the surface area of the anode isoften only about a quarter of the surface area of the cathode. Such acondition favors the rapid formation of Cr(III). When the Cr(III)content exceeds about 11 grams per liter (g.p.l.), a drop in bathconductivity occurs, requiring a higher electromotive force (EMF) inorder to maintain the current density. A high Cr(l'II) content alsoleads to grayness in the chromium plate, and the development of nodulesas the thickness of the plate is increased. In any application ofchromium plating of the latter type, control of the trivalent chromiumin chromic acid baths becomes particularly important.

Description of the prior art In the past, various methods have been usedor sug gested to overcome this problem of excessive Cr(III)concentration. Paul Morisset, Chromium Plating, Robert Draper, Ltd.(1954) refers to a so-called dechromator,

designed to reduced the Cr(III) concentration. This de- I vice employs asmall cathode in a porous container which is hung on the cathode bar ofan electrolytic chromium plating cell employing a large anode area andoperating at high current density. In operation, the Cr(III) in the bulkof the solution is oxidized to the Cr(VI) form while that formed at thecathode is retained in the porous container. Whereas this device offersa partial solution, it is still used in conjunction with an anode oflarge area. R. Seegmiller and V. A. Lamb, (Proc. A.E.S., 1948, pp.125-132), also investigated reoxidation of Cr(III) empolying a chromiumplating bath at a high cathode and low anode current density, whichagain, is the result of using an anode with a large surface arearelative to that of the cathode. A bath temperature of C. isrecommended. The times required to reduce the Cr(IH) content to areasonable level are impractically long (about 22 hours at 80 C. to asmuch as 126 hours at 20 C.). From the foregoing it would appear thatreoxidation of Cr(IH) is time consuming and that it cannot be carriedout under those plating conditions previously discussed, which lead tohigh Cr(IH) concentrations. Although in the past it had been known thatanodes in a plating bath, having a coating of lead dioxide, wouldoxidize Cr(IH), this oxidation had always been carried out in anelectroplating cell, and the oxidation was considered to be es"sentially an electrolytic oxidation.

We have now found that the addition of (PbO to a chromium plating bathcontaining Cr(III) will oxidize it to Cr(VI), and surprisingly, that itwill do so much more rapidly than is possible by the electrolyticmethod, employing the PhD; coating on the anode, even when the surfacearea of the anode exceeds that of the cathode by a considerable amount.The addition of PbO to the bath therefore unexpectedly provides a methodof rapidly reducing the Cr(III) content to an acceptable level. Wheneverthe concentration of Cr (III) is of critical importance, additions ofPbO may offer the only means of control.

This oxidizing action of the PbO is entirely independent of the flow ofcurrent and may be used in those situations where the anode surface isunavoidably less than that of the cathode, and conditions are thereforeconducive to the production of the undesirable Cr(HI).

SUMMARY OF THE INVENTION In accordance with the process of the presentinvention, the quantity of Cr(III) in hexavalent chromium processingsolutions is reduced by the method comprising the steps of:

(a) adding PbO to the solution;

(b) agitating the solution at a temperature of at least 40 C. until theCr(III) in the solution is reduced to the desired concentration; and

(c) removing resulting insoluble lead compounds from the solution.

The method of this invention, as contrasted with prior art methods,provides a rapid and elfective means of reducing the amount of Cr(III)in chromium processing solutions to acceptable levels without theintroduction of impurities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of our invention isapplicable to any hexavalent chromium processing solution wheretrivalent chromium is present, or formed in situ, in undesirablequantities. Typical of such processing solutions are chromium platingbaths to which the present invention is particularly directed. Thefollowing description of the preferred embodiments has therefore beendirected to chromium plating baths, although it should be understoodthat the process may be similarly applied to any hexavalent chromiumprocessing solutions where it is desirable to control and reduce thequantity of trivalent chromium by chemical oxidation, such as chromicacid anodizing solutions.

In carrying out the process of our invention, a quantity of Pb is addedto a hexavalent chromium processing solution sufficient to converttrivalent chromium to hexavalent chromium. The solution is agitated at atemperature of at least 40 C. until the desired trivalent chromiumcontent in the solution is reached, and the resulting insoluble leadcompounds are removed from the solution. An excess of Pb02 above thestoichiometric amount in terms of the reaction between Pb0 and Cr(III)would not be detrimental to a plating bath, but it would serve no usefulpurpose and would raise the cost of the treatment. The addition of anyquantity of PIbO; less than the stoichiometric amount wouldcorrespondingly reduce the amount of Cr(III) present but the amount ofreduction would obviously be limited by the quantity of PbO introduced.Preferably, the PbOg added is essentially the stoichiometric quantitybased on the oxidation reaction:

The plating bath is maintained at a temperature of at least 40 C.,generally at about 40 to 90 C., wth a temperature of about 65 C. to 75C. being preferred. The solution is agitated until the concentration oftrivalent chromium has dropped to an acceptable level. Since PbO' is adense, rapidly settling powder, the degree of agitation should besufficiently vigorous as to keep the PbOg in suspension. The period ofagitation required to reduce the Cr(HI) to an acceptable level may varybetween 5 and 120 minutes with about 50-70 minutes being required at abath temperature of 65-75 C.

The concentration of Cr(III) in the plating bath should generally notexceed about 11 g.p.l. The total concentration of iron and trivalentchromium, however, should generally not exceed about 22 g.p.l. as Fe.Thus, if

no iron is present, the maximum allowable level of Cr(IH) would be about11 g.p.l. If the iron concentration was 15 g.p.l., then the allowableCr(HI) concentration would generally be about 7 g.p.l. Theselimitations, however, are suggested as a guide. The plating technologistwill, through experience, determine the maximum level of Cr(III) whichhe considers tolerable.

Following the treatment of the bath with P1302 the solution is separatedfrom insolubles comprising accumulated lead compounds by decantation,filtration or other conventional means.

In a specific embodiment of the present invention, the bath liquor canbe continuously treated during the plating operation so as to maintainthe Cr(III) content at an acceptable level, thus providing a bath havinggood electrical conductivity and capable of providing a chromiumelectroplate free of the defects generally attributable to an excessiveamount of Cr(III). This may be accomplished by circulating the chromiumplating solution through a bed of lead dioxide. The quantity of leaddioxide is not critical since even a small charge of PbO will oxidize aquantity of Cr(III) commensurate with the amount of said charge. Inorder to provide a bed which will function for an appreciable timewithout replenishing the P130 however, a quantity of PbO: in excess ofthe stoichiometric amount is preferred.

The plating bath solution is continuously circulated through the PbO bedand returned to the plating bath. The temperature of that portion of thebath solution which is circulated through the bed is at least 40 C.,generally about 40 to C., with a temperature of 65 to 75 C. beingpreferred. In order to minimize the reaction time when using thisprocedure, the plating bath solution is prewarmed to 65 75 C. andmaintained at this temperature while circulating through the bed, thenpreferably cooled to essentially the plating bath temperature (generallybetween 45 and 55 C.), and returned to the plating bath.

The PbO bed may take many forms, it may comprise a column requiring noagitation beyond the circulation of the plating bath solution throughthe bed, or it may consist of a vessel wherein the rapidly settling leaddioxide is kept suspended either by the incoming stream of bathsolution, forcefully pumped into the vessel or by a rotating impeller orboth. With such an agitated PbO- bed, separation of the PbO from therecirculating solution may be accomplished by allowing the suspension topass through a quiescent zone wherein the PbO settles back to theagitated bed, and an essentially clear solution returns to the platingbath. A clarifying filter may be included in the system if desired. Byarranging two beds which may be alternately valved into the system,maintenance of a low Cr (III) content in the plating bath may beeffected by one bed While the other is being recharged.

The use of lead dioxide as a chemical oxidant offers several advantagesover other oxidants; for, unlike most, it can be used in the mannerdescribed without introducing contaminants into the plating bath.Although oxidants such as permanganates and persulfates are efiective,they introduce unwanted impurities. In the latter case, their use mayraise the sulfate level above that required for acceptable chromiumelectroplates. Furthermore, an excess of sulfate is contraindicatedwhere it is important to reduce the Cr(III) content, and the excesssulfate would have to be precipitated (as, for example, with bariumcarbonate). With any of these oxidants, the plating operation cannot becarried on while the trivalent chromium is being oxidized.

Another major benefit that may be derived from the use of PhD; is thatcation exchange resins may be employed for the removal of contaminantssuch as copper and iron. The reduction of trivalent chromium by the useof PbO would improve the efficiency of the resin and make ion exchangemore economically attractive than would otherwise be the case.

The following specific examples further illustrate our invention. Partsare by weight except as otherwise noted.

EXAMPLE 1 This example is carried out to experimentally demonstrate thereaction kinetics involved in the reduction of Cr(III) with PbOg- Aportion of a typical chromium plating bath having an initial Cr(III)concentration of 8 g.p.l. is first maintained at 0 C. and thestoichiometric weight of Pb0 (55 grams), is added to it. The mixture isagitated for 30 minutes, then cooled rapidly to room temprature and thesupernatant liquor decanted and analyzed for its Cr(III) content. (Oneanalytical method for determining the Cr(III) content in a chromiumplating bath is a volumetric method employing standard solutions of 0.1N Ce(SO in sulfuric acid, and 0.05 N NaNO The method is derived fromWillard and Young, Trans. Electrochemical Soc. 67 (preprint) 1935.) Thesame procedure is repeated at the same temperature and with portions ofthe same typical plating bath, for a period of 60 minutes, and again fora period of 120 minutes. The results are plotted to produce Curve A ofthe accompanying drawing. Next, a series of determinations are madecovering the same time intervals but with the solution maintained at 70C. Finally a third series of determinations is similarly carried out at90 C. The curves therfore trace the changes in the Cr(III) concentrationwith time and temperature. At 50 C. the reaction rate is found to bequite slow and after 120 minutes the concentration of- Cr(III) hasdecreased from 8 to 4.6 g.p.l. At 70 and 90 C. the reaction rates aremuch faster, and the curves are essentially superimposed, indicatingthat the reaction rates at 70 and 90 C. are substantially the same. Inboth instances, after 1 hour the concentration of Cr(III) has decreasedto about 1.5 g.p.l. The reoxidation rate constantly decelerates so thatafter an additional hour the concentration of Cr(III) has decreased byonly 0.5 g.p.l. At 70 0., therefore, the concentration of Cr(III) can bebrought to an acceptable level within 60 minutes.

EXAMPLE 2 100 gals. (378 liters) of a chromium plating bath conainting250 g.p.l. chromic anhydride (CrO 1.0 g.p.l. sulfate (SO and 2.0 g.p.l.fiuosilicate (SiF is found by analysis to contain 14 g.p.l. Cr(III). Thebath is heated to 70 C. and 80 lbs. (36.2 kilos) of lead oxide (P1302)are stirred in. The bath is agitated for 60 minutes at 70 C., separatedfrom the lead oxide by decantation and cooled to an operatingtemperature of about 50 C. Analysis of the clear bath liquor indicates aCr(III) content of 4.9 g.p.l.

EXAMPLE 3 The same bath as above is put into essentially continuousplating service in a situation wherein the area of the anode is onlyabout one-fourth of that of the cathode, the arrangement being conduciveto the formation of a relatively high concentration of Cr(III). Aportion of the bath liquor is continuously withdrawn while the platingbath is in operation, at the rate of about three gallons per minute, bymeans of a pump. It is passed through a heat exchanger wherein thetemperature is raised to 70 C., then into a thoroughly agitated bedcomprising 1.5 times the stoichiometric quantity of PbO also maintainedat about 70 C. The solution then passes into a quiescent zone whereinessentially all of the PbO powder settles back to the bed and thesolution passes through a second heat exchanger wherein the temperatureis dropped to 50 C., the working temperature of the 6 bath. Theessentially clear solution is continuously returned to the bath.

Two such PbO beds are used alternately so that as the concentration ofCr(IH) builds above the limit chosen (11 g.p.l.), the second bed isvalved into the system, and the first cut out for recharge. Analysis ofthe bath liquor indicates a fairly constant Cr(IH) content of 5 g.p.l.As the bed becomes exhausted, the Cr(III) content of the plating bathslowly climbs to 11 g.p.l. at which point the bed is valved out ofservice.

Although certain preferred embodiments of the invention have beendisclosed for purposes of illustration, it will be evident that variouschanges and modifications may be made therein without departing from thescope and spirit of the invention.

We claim:

1. A method for maintaining the trivalent chromium content of an acidchromium electroplating bath within a concentration range of from about1 to 11 grams per liter, which consists essentially of the steps ofcontacting said bath with an amount of lead dioxide suflicient toconvert said trivalent chromium to hexavalent chromium while maintainingsaid bath while in contact with said lead dioxide at a temperature from40 to 90 C. until the desired trivalent chromium content of the bath isattained, and removing resulting insoluble lead compounds from thebath.-

2. The method of claim 1 wherein the temperature of the solution ismaintained between 65 and C.

3. The method of claim 1 wherein the amount of lead dioxide used isessentially the stoichiometric amount with respect to trivalent chromiumpresent in the bath.

4. The method of claim 1 wherein the process is carried out continuouslyon a portion of the acid chromium electroplating bath withdrawing fromsaid bath, passing said portion through a bed of lead dioxide andreturning said treated portion to said bath.

5. The method of claim 4 in which the plating solution is circulatedthrough the lead dioxide bed while the plating bath is in operation.

6. The method of claim 4 in which the portion of the bath withdrawn isadjusted to between 40 and C. before entering the lead dioxide bed andis readjusted to substantially the temperature of the plating bathbefore being returned to said bath 7. The method of claim 4 in which theportion of the bath withdrawn is warmed to between 65 and 75 C., beforeentering the lead dioxide bed and is cooled to substantially thetemperature of the plating bath before being returned to said bath.

8. The method of claim 4 wherein the bed of lead dioxide is agitated inthe presence of the portion of the plating bath being treated.

9. The method of claim 4 wherein the lead dioxide bed contains at leastthe stoichiometric quantity of PhD; with respect to the Cr(III) presentin the bath.

References Cited UNITED STATES PATENTS 743,668 11/1903 Suchy et a1.204-89 2,600,171 6/1952 Sagen 204-51 2,708,618 5/ 1955 Schwenzfeier23-57 X FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,751,347 I Dated August 7, 1973' Inventofls) John Kraljic, Winslow H.Hartford & Millard F. Good It is certified that error appears in theabove-#"identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 4, Column 6 line 35, "withdrawing" should be withdrawn Signed ands ealed this 27th day of November 1973.

(SEAL) Attest':

EDWARD MELBTCHERJR. RENE TEGTTY/IIZYER Attesting Officer Actifigcoamnissioner of Patents

